The finding heralds the first direct confirmation of this kind of
magma layer at Io and explains why the moon is the most volcanic
object known in the solar system. The research was conducted by
scientists at the University of California, Los Angeles, the
University of California, Santa Cruz, and the University of Michigan.
The study is published this week in the journal Science.

"Scientists are excited we finally understand where Io's magma is
coming from and have an explanation for some of the mysterious
signatures we saw in some of the Galileo's magnetic field data," said
Krishan Khurana, lead author of the study and former co-investigator
on Galileo's magnetometer team at UCLA. "It turns out Io was
continually giving off a 'sounding signal' in Jupiter's rotating
magnetic field that matched what would be expected from molten or
partially molten rocks deep beneath the surface."

Io produces about 100 times more lava each year than all the volcanoes
on Earth. While Earth's volcanoes occur in localized hotspots like
the "Ring of Fire" around the Pacific Ocean, Io's volcanoes are
distributed all over its surface. A global magma ocean about 20 to 30
miles (30 to 50 kilometers) beneath Io's crust helps explain the
moon's activity.

"It has been suggested that both the Earth and its moon may have had
similar magma oceans billions of years ago at the time of their
formation, but they have long since cooled," said Torrence Johnson, a
former Galileo project scientist based at NASA's Jet Propulsion
Laboratory (JPL) in Pasadena, Calif. He was not directly involved in
the study. "Io's volcanism informs us how volcanoes work and provides
a window in time to styles of volcanic activity that may have
occurred on the Earth and moon during their earliest history."

NASA's Voyager spacecraft discovered Io's volcanoes in 1979, making
that moon the only body in the solar system other than Earth known to
have active magma volcanoes. The energy for the volcanic activity
comes from the squeezing and stretching of the moon by Jupiter's
gravity as Io orbits the largest planet in the solar system.

Galileo was launched in 1989 and began orbiting Jupiter in 1995.
Unexplained signatures appeared in magnetic field data from Galileo
flybys of Io in October 1999 and February 2000. After a successful
mission, the spacecraft intentionally was sent into Jupiter's
atmosphere in 2003.

"During the final phase of the Galileo mission, models of the
interaction between Io and Jupiter's immense magnetic field, which
bathes the moon in charged particles, were not yet sophisticated
enough for us to understand what was going on in Io's interior," said
Xianzhe Jia, a co-author of the study at the University of Michigan.

Recent work in mineral physics showed that a group of rocks known as
"ultramafic" rocks become capable of carrying substantial electrical
current when melted. Ultramafic rocks are igneous in origin, or form
through the cooling of magma. On Earth, they are believed to
originate from the mantle. The finding led Khurana and colleagues to
test the hypothesis that the strange signature was produced by
current flowing in a molten or partially molten layer of this kind of
rock.

Tests showed that the signatures detected by Galileo were consistent
with a rock such as lherzolite, an igneous rock rich in silicates of
magnesium and iron found in Spitzbergen, Sweden. The magma ocean
layer on Io appears to be more than 30 miles (50 kilometers) thick,
making up at least 10 percent of the moon's mantle by volume. The
blistering temperature of the magma ocean probably exceeds 2,200
degrees Fahrenheit (1,200 degrees Celsius).